Shalom Daniel,

The Gemara you are referring to says as follows. If the Mitzvah was repeated in the Torah after Matan Torah, this means that the first time it was mentioned was meant for Bnei Noach, and they are also obligated to fulfill the commandment -- for example, Gezel, Retzichah, and Ever Min ha'Chai. If it was not repeated in the Torah after Matan Torah, this implies that only Yisrael are commanded on this Mitzvah, and not Bnei Noach -- for example, Milah and Gid ha'Nasheh.

The logic is that there can be two reasons for mentioning a Mitzvah before Matan Torah, the official point at which Yisrael are obligated to fulfill the Mitzvos: either because it is meant for Bnei Noach too, and we can know this since it is repeated again, so obviously the first mentioning was meant for Bnei Noach and not for Jews; or because the Mitzvah is only for Yisrael, but still, there is a reason for it to be mentioned in an earlier place. The reason for that can be either because Avraham was also commanded on the Mitzvah at that time, or because the Mitzvah is commemorating a certain fact that happened at that time, like Gid ha'Nasheh, which commemorates the incident of Yakov and the Mal'ach. The Torah sometimes mentions Mitzvos in their "natural place" even though the reason Yisrael are obligated to fulfill them is because we were officially commanded to do so at Har Sinai. Another example for this idea is Shabbas. Chazal teach us that in Mara, Moshe taught Yisrael about Shabbas, and still, the commandment to observe the Shabbas is in the Aseres ha'Dibros.

Best Regards,

Aharon Steiner